Past Kinetics AP Free-Response Questions
Link to free-response AP problems from past 10 years: http://www.collegeboard.com/student/testing/ap/chemistry/samp.html Mechanisms 2009 Form A Question 3) The following mechanism has been proposed for the reaction of methane gas with chlorine gas. All species are in the gas phase. Step 1 Cl2 <--> 2 Cl fast equilibrium Step 2 CH4 + Cl → CH3 + HCl slow Step 3 CH3 + Cl2 → CH3Cl + Cl fast Step 4 CH3Cl + Cl → CH2Cl2 + H fast Step 5 H + Cl → HCl fast (d) In the mechanism, is CH3Cl a catalyst, or is it an intermediate? Justify your answer. (e) Identify the order of the reaction with respect to each of the following according to the mechanism. In each case, justify your answer. (i) CH4''(g)'' (ii) Cl2''(g)'' Rates/Rate Laws Problem 1 (2011 Form A Question 6) In an experiment, all the air in a rigid 2.0 L flask is pumped out. Then some liquid ethanol is injected into the sealed flask, which is held at 35°C. The amount of liquid ethanol initially decreases, but after five minutes the amount of liquid ethanol in the flask remains constant. Ethanol has a boiling point of 78.5°C and an equilibrium vapor pressure of 100 torr at 35°C. (a) When the amount of liquid ethanol in the flask is constant, is the pressure in the flask greater than, less than, or equal to 100 torr? Justify your answer. (b) The flask is then heated to 45°C, and the pressure in the flask increases. In terms of kinetic molecular theory, provide TWO reasons that the pressure in the flask is greater at 45°C than at 35°C. In a second experiment, which is performed at a much higher temperature, a sample of ethanol gas and a copper catalyst are placed in a rigid, empty 1.0 L flask. The temperature of the flask is held constant, and the initial concentration of the ethanol gas is 0.0100 M. The ethanol begins to decompose according to the chemical reaction represented below. CH3CH2OH(g) --> CH3CHO(g) + H2(g) The concentration of ethanol gas over time is used to create the three graphs below. © Given that the reaction order is zero, one, or two, use the information in the graphs to respond to the following. (i) Determine the order of the reaction with respect to ethanol. Justify your answer. (ii) Write the rate law for the reaction. (iii) Determine the rate constant for the reaction, including units. (d) The pressure in the flask at the beginning of the experiment is 0.40 atm. If the ethanol completely decomposes, what is the final pressure in the flask? Problem 2 8 H+ (aq) + 4 Cl− (aq) + MnO4− (aq) → 2 Cl2 (g) + Mn3+ (aq) + 4 H2O (l) 3. Cl2 (g) can be generated in the laboratory by reacting potassium permanganate with an acidified solution of sodium chloride. The net-ionic equation for the reaction is given above. (a) A 25.00 mL sample of 0.250 M NaCl reacts completely with excess KMnO4 (aq). The Cl2 (g) produced is dried and stored in a sealed container. At 22°C the pressure of the Cl2 (g) in the container is 0.950 atm. (i) Calculate the number of moles of Cl− (aq) present before any reaction occurs. (ii) Calculate the volume, in L, of the Cl2 (g) in the sealed container. An initial-rate study was performed on the reaction system. Data for the experiment are given in the table below. (b) Using the information in the table, determine the order of the reaction with respect to each of the following. Justify your answers. (i) Cl- (ii) MnO4- © The reaction is known to be third order with respect to H+. Using this information and your answers to part (b) above, complete both of the following: (i) Write the rate law for the reaction. (ii) Calculate the value of the rate constant, k, for the reaction, including appropriate units. (d) Is it likely that the reaction occurs in a single elementary step? Justify your answer. 2008 A – 3d-f – finding rate laws 2008 B - #2 rate laws and mechanisms 2005 A - #3 rate laws 2005 B - #3 rate laws 2004 A - #3 rate laws 2004 B - #3 Rate laws 2003 A - #3 Rate laws/half life 2003 B - #8 Rate laws/half life